This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The objective of this study was to investigate effects of organic methyl mercury (MeHg) ingestion on the fertility rate of female rats, including (1) the number of viable fetuses, (2) the number of implantations, and the number of corpora lutea. Female rats, 30 days of age, were treated with 2.5 ppm (low treatment group) or 5.0 ppm (high treatment group) or 0.0 ppm MeHg (control group), in drinking water, for 60 days. They were then mated with 70 to 80-day-old untreated males (2 females to 1 male) for 17 days. Pregnancy was confirmed by weight increase. At 12-15 days of gestation, all females were euthanized and examined for the number of viable fetuses and implantations. Compared to controls, neither the number of viable fetuses, nor the number of implantations, and nor the number of corpora lutea was altered, as a result of MeHg treatment. Conversely, as reported previously in 2006-07 progress report, the ingestion of inorganic mercury resulted in lowering the mean number of fetuses (6.9 vs 12.4 in controls) as well as implantations (10.5 vs 12.8 in controls). In conclusion, while MeHg does not adversely affect the female reproductive system, inorganic mercury does. The difference between the two may lie in differences in their ability to cross the blood-brain-barrier, which is crossed by the MeHg but not by the inorganic mercury. These results are significant because they have a direct impact on young and adult women, especially child-bearing women, who live near coastal areas and depend upon fish as their main source of diet.